The present invention relates to an improved cryogenic gas separation process.
Mixtures of gases are frequently separated into the component gases by cryogenic techniques. An example is the separation of ethane (and heavier hydrocarbons) from methane. Recent increases in the market for ethane, propane and heavier hydrocarbons have created the need for processes yielding higher recovery of these products.
Several variations of prior art cryogenic separation processes are described in U.S. Pat. No. 4,278,457 issued July 14, 1981 and the present invention will be compared to the processes disclosed in that U.S. patent.
U.S. Pat. No. 4,278,457 deals primarily with the problem of increasing ethane recovery while at the same time reducing the danger of CO.sub.2 icing. This is accomplished by splitting the vapor stream to the demethanizer column into two portions. This was found to reduce the risk of CO.sub.2 icing without increasing column overhead temperature so that the ethane recovery was not adversely effected. The vapors can be split either before or after the preliminary cooling stages. The first portion of the vapor is cooled to substantial condensation, expanded to the column operating pressure and supplied as a column feed usually at the top of the column. The second portion of the vapor is expanded through a work expansion machine. This stream is cooled sufficiently prior to expansion so that the column top temperature can be controlled by the column top feed. The column refrigeration is provided by the combined cooling effect of the first and second portions of the split vapor feed. Any condensed liquids that result can be expanded and supplied as a lower mid-column feed.